Project 3: An Introduction
to File Systems
CS3430
Operating Systems
University of Northern Iowa
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Introduction
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The goal of project 3 is to understand
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basic file system design and implementation
file system testing
data serialization/de-serialization
At the end of the project, you will feel like a
file system expert!
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Outline
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Project 3
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Specification
Downloading and testing file system image
General FAT32 data structures
Endian-ness
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Project 3
More than you wanted to know about
FAT32..
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Project 3
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You will create a user-space utility to
manipulate a FAT32 file system image
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No more kernel programming!
Utility must understand a few basic
commands to allow simple file system
manipulation
Utility must not corrupt the file system and
should be robust
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FAT32 Manipulation Utility
Utility only recognizes the following built-in readonly commands:
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info
stat
volume
quit
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cd
ls
read
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File System Image
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Manipulation utility will work on a preconfigured FAT32 file system image
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Actually a file
File system image will have raw FAT32 data
structures inside
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Just like looking at the raw bytes inside of a disk
partition
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File System Image
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Your FAT32 manipulation utility will have to
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Open the FAT32 file system image
Read parts of the FAT32 file system image and
interpret the raw bytes inside to service your
utility’s file system commands…
…just like a file system!
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General FAT32 Data
Structures
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Terminology
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Byte – 8 bits of data, the smallest
addressable unit in modern processors
Sector – Smallest addressable unit on a
storage device. Usually this is 512 bytes
Cluster – FAT32-specific term. A group of
sectors representing a chunk of data
FAT – Stands for file allocation table and is a
map of files to data
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FAT32 Disk Layout
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3 main regions…
Reserved
Region
Track
Sector
FAT
Region
Data
Region
Disk arm
Reserved Region
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Reserved Region – Includes the boot
sector, the extended boot sector, the file
system information sector, and a few other
reserved sectors
Reserved
Region
Boot Sector
FS Information
Sector
FAT
Region
Additional
Reserved Sectors
(Optional)
Data
Region
FAT Region
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FAT Region – A map used to traverse the
data region. Contains mappings from cluster
locations to cluster locations
Reserved
Region
FAT
Region
File Allocation Table #1
Data
Region
Copy of File Allocation
Table #1
Data Region
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Data Region – Using the addresses from
the FAT region, contains actual file/directory
data
Reserved
Region
FAT
Region
Data
Region
Data until end of partition
Endian
Big or little?
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Machine Endianness
The endianness of a given machine
determines in what order a group of bytes
are handled (ints, shorts, long longs)
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Big-endian – most significant byte first
Little-endian – least significant byte first
This is important to understand for this
project, since FAT32 is always formatted as
little-endian
FAT32 Endianness
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The following are a few cases where
endianness matters in your project:
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Reading in integral values from the FAT32
image
Reading in shorts from a FAT32 image
Combining multiple shorts to form a single
integer from the FAT32 image
Interpreting directory entry attributes
Endian Example (English Version)
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Imagine you can only communicate three
letters at a time, and your word is “RAPID”
Big-endian
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1. RAP
2. ID
Word = RAPID
Little-endian
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1. PID
2. RA
Word = PIDRA (come again?)
Endian Example (data version)
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short value = 15; /* 0x000F */
char bytes[2];
memcpy(bytes, &value, sizeof(short));
In little-endian:
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bytes[0] = 0x0F
bytes[1] = 0x00
In big-endian:
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bytes[0] = 0x00
bytes[1] = 0x0F
Endian Example (data version 2)
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int value = 13371337; /* 0x00CC07C9 */
char bytes[4];
memcpy(bytes, &value, sizeof(int));
In little-endian:
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In big-endian:
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bytes[0] = 0xC9
bytes[1] = 0x07
bytes[2] = 0xCC
bytes[3] = 0x00
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bytes[0] = 0x00
bytes[1] = 0xCC
bytes[2] = 0x07
bytes[3] = 0x09
Visualizing Example 2
Value = 13371337 (0x00CC07C9)
index
0
1
2
3
little
endian
0xC9
0x07
0xCC
0x00
big
endian
0x00
0xCC
0x07
0xC9
Helper Resources and Programs
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Look at the resources page to see how to do file
I/O in C
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Look at how to compile python and java
programs on the Linux virtual machines
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Will have to read and seek around in the image file.
If your project does not compile or run on the Linux
virtual machines, the most it can earn is 50%
Parameter passing program
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In C, but other languages are very similar
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Additional Project 3 Information
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Like other projects, may work in teams of 2 or
alone
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Next Steps
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Take a look at the fat32 specification file from
Microsoft
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Somewhat confusing – just like the real world
(haha)
Take a look at the image file
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